The invention relates to radio receivers having the capability of receiving digital television (DTV) signals, no matter whether they are transmitted using quadrature amplitude modulation (QAM) of the principal carrier wave or they are transmitted using vestigial sideband (VSB) amplitude modulation of the principal carrier wave.
A Digital Television Standard published Sep. 16, 1995 by the Advanced Television Systems Committee (ATSC) specifies vestigial sideband (VSB) signals for transmitting digital television (DTV) signals in 6-MHz-bandwidth television channels such as those currently used in over-the-air broadcasting of National Television System Committee (NTSC) analog television signals within the United States. The VSB DTV signal is designed so its spectrum is likely to interleave with the spectrum of a co-channel interfering NTSC analog TV signal. This is done by positioning the pilot carrier and the principal amplitude-modulation sideband frequencies of the DTV signal at odd multiples of one-quarter the horizontal scan line rate of the NTSC analog TV signal that fall between the even multiples of one-quarter the horizontal scan line rate of the NTSC analog TV signal, at which even multiples most of the energy of the luminance and chrominance components of a co-channel interfering NTSC analog TV signal will fall. The video carrier of an NTSC analog TV signal is offset 1.25 MHz from the lower limit frequency of the television channel. The carrier of the DTV signal is offset from such video carrier by 59.75 times the horizontal scan line rate of the NTSC analog TV signal, to place the carrier of the DTV signal about 309,877.6 kHz from the lower limit frequency of the television channel. Accordingly, the carrier of the DTV signal is about 2,690122.4 Hz from the middle frequency of the television channel. The exact symbol rate in the Digital Television Standard is (684/286) times the 4.5 MHz sound carrier offset from video carrier in an NTSC analog TV signal. The number of symbols per horizontal scan line in an NTSC analog TV signal is 684, and 286 is the factor by which horizontal scan line rate in an NTSC analog TV signal is multiplied to obtain the 4.5 MHz sound carrier offset from video carrier in an NTSC analog TV signal. The symbol rate is 10.762238*106 symbols per second, which can be contained in a VSB signal extending 5.381119 MHz from DTV signal carrier. That is, the VSB signal can be limited to a band extending 5.690997 MHz from the lower limit frequency of the television channel.
The ATSC standard for digital HDTV signal terrestrial broadcasting in the United States of America is capable of transmitting either of two high-definition television (HDTV) formats with 16:9 aspect ratio. One HDTV format uses 1920 samples per scan line and 1080 active horizontal scan lines per 30 Hz frame with 2:1 field interlace. The other HDTV format uses 1280 luminance samples per scan line and 720 progressively scanned scan lines of television image per 60 Hz frame. The ATSC standard also accommodates the transmission of DTV formats other than HDTV formats, such as the parallel transmission of four television signals having normal definition in comparison to an NTSC analog television signal.
DTV transmitted by vestigial-sideband (VSB) amplitude modulation (AM) during terrestrial broadcasting in the United States of America comprises a succession of consecutive-in-time data fields each containing 313 consecutive-in-time data segments. There are 832 symbols per data segment. So, with the symbol rate being 10.76 MHz, each data segment is of 77.3 microseconds duration. Each segment of data begins with a line synchronization code group of four symbols having successive values of +S, xe2x88x92S, xe2x88x92S and +S. The value +S is one level below the maximum positive data excursion, and the value xe2x88x92S is one level above the maximum negative data excursion. The initial line of each data field includes a field synchronization code group that codes a training signal for channel-equalization and multipath suppression procedures. The training signal is a 511-sample pseudo-random noise sequence (or xe2x80x9cPN-sequencexe2x80x9d) followed by three 63-sample PN sequences. The middle 63-sample PN sequence is transmitted in accordance with a first logic convention in the first line of each odd-numbered data field and in accordance with a second logic convention in the first line of each even-numbered data field, the first and second logic conventions being one""s complementary respective to each other.
The subsequent lines of each data field contain data that have been Reed-Solomon forward error-correction coded. In over-the-air broadcasting the error-correction coded data are then trellis coded using twelve interleaved trellis codes, each a 2/3 rate trellis code with one uncoded bit. Trellis coding results are parsed into three-bit groups for over-the-air transmission in eight-level one-dimensional-constellation symbol coding, which transmission is made without symbol pre-coding separate from the trellis coding procedure. Trellis coding is not used in cablecasting. The error-correction coded data are parsed into four-bit groups for transmission as sixteen-level one-dimensional-constellation symbol coding, which transmissions are made without precoding.
The VSB signals have their natural carrier wave, which would vary in amplitude depending on the percentage of modulation, suppressed. The natural carrier wave is replaced by a pilot carrier wave of fixed amplitude, which amplitude corresponds to a prescribed percentage of modulation. This pilot carrier wave of fixed amplitude is generated by introducing a direct component shift into the modulating voltage applied to the balanced modulator generating the amplitude-modulation sidebands that are supplied to the filter supplying the VSB signal as its response. If the eight levels of 4-bit symbol coding have normalized values of xe2x88x927, xe2x88x925, xe2x88x923, xe2x88x921, +1, +3, +5 and +7 in the carrier modulating signal, the pilot carrier has a normalized value of 1.25. The normalized value of +S is +5, and the normalized value of xe2x88x92S is xe2x88x925.
VSB signals using 8-level symbol coding will be used in over-the-air broadcasting within the United States, and VSB signals using 16-level symbol coding are proposed in the ATSC standard for use in over-the-air narrowcasting systems or in cable-casting systems. However, the standard practice in such systems is to use suppressed-carrier quadrature amplitude modulation (QAM) signals rather than VSB signals. This presents television receiver designers with the challenge of designing receivers that are capable of receiving either type of transmission and of automatically selecting suitable receiving apparatus for the type of transmission currently being received. The design of such receiving apparatus with intermediate-frequency (IF) amplifiers used in common for both QAM and VSB signals has been described by C. B. Patel and the inventor in their U.S. Pat. No. 5,506,636 issued Apr. 9, 1996, entitled HDTV SIGNAL RECEIVER WITH IMAGINARY-SAMPLE-PRESENCE DETECTOR FOR QAM/VSB MODE SELECTION, and incorporated herein by reference. This type of QAM/VSB DTV receiver is also described by C. B. Patel and the inventor in U.S. patent application Ser. No. 08/266,753 filed Jun. 28, 1994 and entitled RADIO RECEIVER FOR RECEIVING BOTH VSB AND QAM DIGITAL HDTV SIGNALS, U.S. Pat. No. 5,715,012 issued Feb. 3, 1998 and entitled RADIO RECEIVERS FOR RECEIVING BOTH VSB AND QAM DIGITAL HDTV SIGNALS, and in U.S. patent application Ser. No. 08/773,949 filed Dec. 26, 1996 and entitled RADIO RECEIVERS FOR RECEIVING BOTH VSB AND QAM DIGITAL HDTV SIGNALS. U.S. Pat. Nos. 5,506,636 and 5,715,012 and U.S. patent application Ser. No. 08/266,753 were written presuming that the carrier frequency of a VSB DTV signal would be 625 kHz above lowest channel frequency, as earlier proposed by a subcommittee of the Advanced Television Systems Committee. This specification presumes that the carrier frequency of a VSB DTV signal is nominally 310 kHz above lowest channel frequency, as specified in Annex A of the Digital Television Standard published Sep. 16, 1995.
In U.S. patent application Ser. No. 08/826,790 filed Mar. 24, 1997 and entitled DTV RECEIVER WITH FILTER IN I-F CIRCUITRY TO SUPPRESS FM SOUND CARRIER OF NTSC CO-CHANNEL INTERFERING SIGNAL the inventor describes the desirability of using trap filtering for NTSC sound in the intermediate-frequency amplifiers used for VSB DTV signals in order to facilitate the suppression of co-channel interference from NTSC analog TV signals. Trap filtering for NTSC sound can also be used in the intermediate-frequency amplifiers used for QAM DTV signals, but is not usually considered necessary. It is difficult to provide trap filtering for NTSC sound without some phase distortion in the passband close to the frequencies the trap filter suppresses and accordingly it may be desirable to avoid the use of the trap filtering in the IF amplifiers for QAM signals.
It may also be possible to reduce co-channel interference from NTSC video carrier and chroma subcarrier by trap filtering at those frequencies in the IF amplifiers for VSB signal without introducing unacceptable error into the VSB symbol coding. If such trap filtering is feasible, it would be desirable to avoid the use of this trap filtering in the IF amplifiers for QAM signals.
In reception apparatus having IF amplifiers used for QAM DTV signals and having separate IF amplifiers used for VSB DTV signals, it is preferable to continue synchrodyning to baseband in the digital regime, rather than in the analog regime. That is, the response of the response of the final IF amplifier for VSB DTV signals is still preferably synchrodyned to baseband in the digital regime as described by C. B. Patel and the inventor in their U.S. Pat. No. 5,479,449 issued Dec. 26, 1995 and entitled DIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER, AS FOR INCLUSION IN AN HDTV RECEIVER; and the response of the final IF amplifier for QAM DTV signals is still preferably synchrodyned to baseband in the digital regime as described in U.S. Pat. No. 5,715,012.
These synchrodyning procedures are still preferably carried out using final intermediate frequencies that place the carriers of the QAM and VSB signals at submultiples of the sampling rate used during the digitization of these frequencies, which sampling rate is preferably a harmonic of the symbol rate. This facilitates the storage of digital descriptions of the final intermediate frequencies in digital memory, for use in the digital synchrodyning procedures.
A radio receiver for receiving a selected digital television signal, irrespective of whether it is a quadrature-amplitude-modulation (QAM) or a vestigial-sideband (VSB) digital television signal, takes the following form in the invention. A first intermediate-frequency amplifier chain is connected for supplying a first final intermediate-frequency response to the selected digital television signal. A first analog-to-digital converter is connected for digitizing the first final intermediate-frequency response to generate a digitized first final intermediate-frequency response. QAM synchrodyning circuitry is connected for generating real and imaginary sample streams of interleaved QAM symbol code by synchrodyning the digitized first final intermediate-frequency response to baseband providing it is a QAM signal. A second intermediate-frequency amplifier chain is connected for supplying a second final intermediate-frequency response to the selected digital television signal. A second analog-to-digital converter is connected for digitizing the second final intermediate-frequency response to generate a digitized second final intermediate-frequency response. VSB synchrodyning circuitry is connected for generating a real sample stream of interleaved VSB symbol code by synchrodyning the digitized second final intermediate-frequency response to baseband providing it is a VSB signal.
In certain preferred embodiments of the invention the second intermediate-frequency amplifier chain is constructed so the second final intermediate-frequency response is more selective than the first final intermediate-frequency response, so as to exclude substantial response to most of the audio carrier of the accompanying co-channel analog television signal, if such co-channel analog television signal obtains.